Mission of NOAA's NWS Hydrologic Services Program

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Mission of NOAA's NWSMission of NOAA's NWSHydrologic Services ProgramHydrologic Services Program

To provide river and flood forecasts and warnings for protection of life and property

Provide basic hydrologic forecast information for the nation's economic and environmental well being.


1) Flash Flood Products…• Are issued for life-threatening floods which occur in <6 hrs from

causative event!! • Floods which require immediate action to protect lives and property:

– Dangerous flooding of streams, washes, alluvial fans (overland flooding) or urban areas…due to extremely heavy rain.

– Dam or levee failures…can happen anytime, even a clear day or night!

• Products:– Flood Watch (in Header): FFA

• “Flash Flood Watch” in Headline• 6-48 hours ahead of event• Original & Follow-Ups

– Flash Flood Warning: FFW• Occurring or Imminent• EAS!

– Flash Flood Statement FFS• Follow-up to FFW.

– UGC: Watch: Zones; Others: Counties


2) Forecast Point Flood Products… • Flood Products Which Include a Quantified

Value (i.e. CNRFC forecast) and Impact Information:– For specific locations;– Applicable to specific reaches (area along river

up and downstream of forecast point);– For flooding which is expected to occur in >6

hrs from causative event.– May be in effect for same time as flash flood or

areal flood products in the encompassing area.– Point Flood Products Should Not Be Combined

with Areal Products (except Flood Outlooks).• Products:

– Flood Outlook (ESF/PNS); 36 hrs – 7 days– Flood Watch (FFA); 6-48 hrs; 72 hrs if likely. – Flood Warning (FLW); <24 hrs– Flood Advisory (FLS); Nuisance Flooding– Flood Statement (FLS); Follow-up to FLW– Recommend EAS for 1st Major Flood Warning – UGC: Counties


3) “Areal” Flood Products:• Issued for flooding which is:

– Occurring or expected in a defined area such as a portion of a state, a group of counties, urban and/or small stream areas, areas along rivers or streams which threaten lives and property.

– Not appropriately covered by Flash Flood or Forecast Point Flood Products.

– For flooding which is expected to occur in >6 hrs from causative event.– May be in effect for same time as flash flood or forecast point flood products in

the encompassing area.– Areal FFAs & FLWs Should Not Be Combined w/Forecast Point FFAs & FLWs;

OK to combine Flood Outlooks.

• Products:– Flood Outlook (ESF/PNS); 36 hrs – 7 days– Flood Watch (FFA); 6-48 hrs; 72 hrs if likely– Flood Warning (FLW); <24 hrs– Flood Advisory (FLS); Nuisance– Flood Statement (FLS); Follow-up to FLW– Recommend EAS for 1st Major Flood Warning– UGC: Watch: Zones; Others: Counties


Flood Outlook:GET READY!

• Flood Outlook (ESF & PNS*)

– Flooding possible in >36 hrs to ~7 days (issue with greatest lead time feasible!).

– Hazardous flooding event may develop. – Intended to provide information to those who need

considerable lead time to prepare for an event.– OK to combine with areal flood outlook– UGC: Counties– Update & end w/another ESF & PNS. – Create w/AWIPS Formatter

• * To get Flood Outlook info on REV Website map, always issue RNOSPSREV in addition to the RNOESFREV.


Flood Watch: GET SET!!

• Flood Watch (FFA) – Flooding possible in 24 to 72 hrs (>48 hrs only if >50%

probability; use greatest lead time feasible).– Expectation of a flood event has increased, but its occurrence,

location, and/or timing is still uncertain. – Intended to provide enough lead time so those who need to set

their mitigation plans in motion can do so.– UGC: Counties (Zones for Areal)– Don’t combine w/Areal Flood Watch– Update & end w/another FFA. – Create w/River Pro (GHG for Areal)– Update Zones.


Flood Warning:GO!!!

• Flood Warning (FLW) – Flooding forecast in <24 hrs (up to 36 hrs, >24 hrs only if from CNRFC), or is

already occurring.– Update & end w/FLS. – Create w/RiverPro (WarnGen for Areal);– UGC: Counties

– EAS Recommended if First Warning for MAJOR Flooding; • EAS Not required for minor or moderate flooding, but may be used per

FIC discretion or EM request, depending on circumstances. – Update Zones.– Don’t combine w/Areal Flood Warning.


Flood Advisory:Minor Flooding Expected;

Be Careful Out There!

• A Flood Advisory (FLS) may be issued (forecaster’s discretion) for Mainstem River Forecast Points, If:– Forecast is for minor flooding only; no significant threat to

life/property;• If flooding which is a threat to life & property is expected (>0.5’

above flood stage), a flood warning (FLW) must be issued!!!– NO further rises are expected thereafter;– Rises mainly from snowmelt; no heavy rain in forecast…

– Don’t combine with Areal Flood Advisory– UGC: Counties– Update & end w/FLS. – Create w/RiverPro On NWR.


Flood Statement: “News” About

Flood Situation…• Flood Statements (FLS):

– Give the current status of previously issued NWS Flood Warning or Advisory.

– No significant change to previously forecast crest stages (<0.5’) or timing (<6 hrs) expected.

– Issued at least twice per day when minor flooding is occurring/expected.

– Issued 3 to 4 times per day when moderate to major flooding is occurring/expected.

– Create w/RiverPro – UGC: Counties– Don’t combine Point w/Areal Flood Statements.


Hydrologic Statements:Currently, It Is Not Expected to Flood, but Significant Rises are Expected…

• Hydrologic Statements (RVS):– Issued when river forecasts have been prepared

for rivers in the HSA, or to disseminate information on significant hydrologic conditions.

– Rivers not currently expected to flood, but significant rises (above Monitor Stage) expected).

– Create w/RiverPro – UGC: Counties– On NWR


River Stage:• Level of the surface of the water at a given

location (in a river, stream or lake) above a datum*. Not the depth above the bottom!

• *Datum: Usually an arbitrary point which has a measured elevation above MSL.


Monitor Stage• The stage at which initial action must be taken by concerned

interests:– Livestock movement, removal of equipment from lowest overflow

areas, or general surveillance of the situation. • May produce limited overbank flows sufficient to cause minor

flooding of low-lying lands and local roads. • Typically ~75% of flood flow.• If reached, a state of readiness must be maintained by those

concerned about possible flooding on that stretch of river.


Flood Stage• Stage set for a particular river gage where

water will just begin to spill out of banks and cause flooding problems somewhere on that stretch of river or stream.

• If Flood Stage is forecast to be exceeded or is actually exceeded, the NWS will issue a Flood Warning for that stretch of river.


Minor, Moderate, Major Flooding:• Minor: Minimal or no property damage, but

some public inconvenience.

• Moderate: Inundation of secondary roads; movement of property to higher elevations necessary to avoid damage. Evacuation of lowest portions of floodplain necessary.

• Major: Extensive inundation and property damage. Evacuation of people, livestock, inventory, equipment necessary. Most primary & secondary roads closed. Impacts to transportation, power, & communications.

Major Flooding

Moderate Flooding

Minor Flooding

Near Flood Stage

No Flooding


Antecedent Soil Moisture Conditions:

• The degree of wetness of the soil at the onset of a rainfall or snowmelt event.

• A primary component of the percentage of rainfall or snowmelt which will run off.


Cubic feet Per Second:

• A unit of measurement of discharge equal to streamflow of one cubic foot of water per second past a given point.

• What most streamflow in the USA is measured in, and what the NWS forecasts…and then usually converts to stage via a rating.

• 1 cubic foot ~= 7.5 gallons, or about the same volume as 4 basketballs.– So, 10,000 cfs ~= 40,000 basketballs full of water per second!


Flood Frequency…• Statistical frequency, expressed in years, at which a flood of a given

magnitude should, on average be equaled or exceeded, given a record of past flood events.

• Equal to 100 divided by the percent chance that a flood will happen any year.

– Example: If a certain flood has a 2% chance of occurring any given year, its frequency would be 100/2 = 50 yr flood.

• As the occurrence of floods is random in time, a flood of any frequency may occur in any year or in successive years.

• A flood of a certain frequency will change as more flood record evolves over time.


Hydrograph• A graph showing river stage and/or stream

flow with respect to time.


Six Largest Floods on the Truckee River at Reno Since 1950…

(Flood Flow = ~10,500 cfs)

1. December 23, 1955….....20,800 cfs2. November 21, 1950….....19,900 cfs3. February 1, 1963……......18,400 cfs 4. January 2, 1997……........18,200 cfs5. December 31, 2005……..16,400 cfs6. February 17, 1986…........14,400 cfs

• All between 50 and 100 year events…– 1% to 2% chance any year…


Six Largest Floods on the Truckee River at Vista Since 1963…

(Flood Flow = ~6,800 cfs) 1. February 1, 1963……......18,900 cfs2. January 2, 1997……........18,500 cfs3. February 17, 1986…........16,100 cfs4. December 31, 2005……..13,700 cfs5. December 23, 1964……..11,700 cfs6. January 14, 1980……….. 9,970 cfs• All between 25 and 100 year events…

– 1% to 4% chance any year…


Key Ingredients for Flooding in the

Reno Hydrologic Services Area (HSA)…

(or…Conditions to Watch Out For)


1) Excessive Rain Over a Large Area, Especially at

Higher Elevations…• Truckee River Floods Usually Begin in the Sierra…• What’s Excessive Rain…??

– Storm Totals: >8-10” rain at Tahoe City/Truckee (6000-6500’); >1” rain at Reno (4400’): ~10 to 1 ratio!!

– High Elevation Rain Rates: • >1”/3hrs, >2”/12hrs , >3”/ 1 day• But…can be much less (i.e., half above amounts or less) if:

• Soils are saturated, • Rain on ripe (ready to melt) snowpack (ripe snow = >0.4 in

water per inch of snow), • Rain on steep, lightly vegetated, impermeable surfaces


16.73

5.04

12.78

4.56

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5.8

13.63

4.65

11.95

5.37

19.64

4.36

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Inches

Dec. 19-24,1955

Nov. 16-21,1950

Jan. 29– Feb. 2, 1963

Dec. 261996-

Jan. 3 1997

Dec. 262005-Jan. 4

2006

Feb. 13-20,1986

Storm Dates

TotalLargest Day

Tahoe City Precipitation Totals; Six Largest Reno Floods Since 1950:


2) High Snow Levels=Flood Producing Runoff…

• The Higher the Snow Level, the Larger the Area Contributing to the Flood…

• Generally, Snow Levels above 7500’ are considered “High” and can cause flood problems…

• If temperatures are higher than ~37 deg at Tahoe City/S Lk Tahoe or ~45 at Reno during a major storm, high flood potential on Truckee R!

• Representative Elevations on Truckee Basin…– 8200-9000’: Mt. Rose Ski Area– 6500-8500’: Alpine Meadows/Squaw Valley– 7200’: Donner Summit– 6300’: Tahoe City…– 5900’: Truckee – 5200’: Farad– 4900’: Verdi– 4400’: Reno

• Heavy rain with relatively low snow levels (i.e. 5500-6500’) can cause flooding on tributaries (e.g. Steamboat Ck., N Truckee Drain), and urban/small stream flooding in valleys.


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3743

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Degrees F

Nov. 16-21, 1950 (6 Days)

Dec. 19-24, 1955 (6 Days)

Jan. 29–Feb. 2,1963 (5 Days)

Feb. 13-20, 1986 (8 Days)

Dec. 26 1996- Jan. 3, 1997 (9)

Storm Dates

Avg. TempWarmest DayMax Temp

Temperature at Tahoe City During Five Largest Truckee River Floods…


Other Conditions Which Have the Potential to Cause or Aggravate Flooding on the

Truckee:

• Large low elevation snowpack (over 6 in. below 6000 ft), esp. if ready to melt (>0.4 inch water per inch of snow). – If not ready to melt, snow is able to absorb a lot of

rain before melting, like a sponge.– Ice and hard packed snow act as an impermeable

surfaces and increases runoff.• River/stream levels already high or rapidly rising.• Saturated or Frozen Soils. • Long duration Moderate (0.1 to 0.3 in/hr) to

Heavy (>0.3 in/hr) rain.


Other Conditions Which Have the Potential to Cause or Aggravate

Flooding on the Truckee:• Channel Capacity:

– Some portions of a river’s channel have significantly lower capacity than others; therefore, flooding may occur at lower flows on some portions of a river than others;

• e.g., channel capacity (flood stage) at Truckee R @ Reno is ~10500 cfs; at Vista it is ~6800 cfs.


Other Conditions Which Have the Potential to Cause or Aggravate

Flooding on the Truckee:• Reservoirs…

– If reservoirs have flood control storage allotted, and are able to be operated during winter storms to control downstream flows, they can greatly decrease flood severity (e.g. Prosser, Boca, Stampede)

– However, once reservoirs with flood control storage fill, they loose their flood control capability.

Mission of NOAA's NWS Hydrologic Services Program

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